Leukemia is a disease in which immature hematopoietic stem cells are developed and proliferate abnormally in bone marrow. Leukemia is classified into 4 types (acute myeloid leukemia, chronic myeloid leukemia, acute lymphocytic leukemia and chronic lymphocytic leukemia) depending on type of developed cells and proliferation rate thereof. Myelodysplastic syndrome is also included in leukemia because of its similarity and correlation in findings.
Acute leukemia is a disease in which hematopoietic stem cells and/or progenitor cells are developed and only specific cells proliferate as leukemia cells. On the other hand, chronic leukemia is a disease in which stem cells per se proliferate autonomously but maintain ability to differentiate into mature cells, and only cells originated from a certain clone proliferate autonomously. Myelodysplastic syndrome is a disease in which hematopoietic stem cells acquire genetic abnormality a posteriori and thereby presenting cytopenia and/or dysplasia in hematopoietic system.